Theory of Spin Hall Effects in Semiconductors

TL;DR

This paper reviews the theoretical foundations of the spin Hall effects in semiconductors, covering mechanisms, equations, and phenomena related to spin transport induced by electric currents without magnetic fields.

Contribution

It provides a comprehensive overview of the theoretical models, mechanisms, and recent developments in understanding spin Hall effects in various semiconductor systems.

Findings

Different mechanisms of spin transport identified: spin precession, skew scattering, side jump.

Relations between spin currents and spin polarization clarified.

Electrically induced spin effects in bulk, boundaries, and mesoscopic systems summarized.

Abstract

Spin Hall effects are a collection of phenomena, resulting from spin-orbit coupling, in which an electrical current flowing through a sample can lead to spin transport in a perpendicular direction and spin accumulation at lateral boundaries. These effects, which do not require an applied magnetic field, can originate in a variety of intrinsic and extrinsic spin-orbit coupling mechanisms and depend on geometry, dimension, impurity scattering, and carrier density of the system--making the analysis of these effects a diverse field of research. In this article, we give an overview of the theoretical background of the spin Hall effects and summarize some of the most important results. First, we explain effective spin-orbit Hamiltonians, how they arise from band structure, and how they can be understood from symmetry considerations; including intrinsic coupling due to bulk inversion or structure asymmetry or due to strain, and extrinsic coupling due to impurities. This leads to different mechanisms of spin transport: spin precession, skew scattering, and side jump. Then we present the kinetic (Boltzmann) equations, which describe the spin-dependent distribution function of charge carriers, and the diffusion equation for spin polarization density. Next, we define the notion of spin currents and discuss their relation to spin polarization. Finally, we explain the electrically induced spin effects; namely, spin polarization and currents in bulk and near boundaries (the focus of most current theoretical research efforts), and spin injection, as well as effects in mesoscopic systems and in edge states.